1. Field of the Invention
The invention relates to a method of computed tomography in which X-rays from an X-ray source irradiate an object to be examined and are detected by means of a position-sensitive X-ray detector array which occupies a substantially fixed position relative to the X-ray source, the X-ray source and the X-ray detector array being moved together in a number of orientations relative to the object to be examined in order to pick up a number of X-ray images, a density distribution of the object being reconstructed by means of the detected X-ray images and unsharpness in the reconstructed image being reduced by means of deconvolution. The invention also relates to an apparatus for carrying out such a method.
2. Description of the Related Art
Such an apparatus and method are known from EP-A 0 353 299, which corresponds to U.S. Pat. No. 5,120,520. The cited publication describes a method of processing data acquired by means of a computed tomography apparatus (CT scanner) in which the X-ray source emits radiation which originates not only from a focal point but also, unintentionally, from the surroundings of the focal point. These undesirable X-rays are caused by scattering of electrons within the X-ray tube and, consequently, the X-rays originate from an extensive area. Because the X-ray source is no longer substantially punctiform, the reconstructed image contains unsharp edges and artefacts. In the known method the image artefacts caused by the source are reduced by deconvolution with a PSF (point spread function) describing the characteristics of the X-ray source.
In CT scanners in which the problem described in EP-A 0 353 299 does not occur, or in which this problem has been adequately mitigated, unsharpness and artefacts still occur in the reconstructed images. Notably a loss of contrast occurs. These image artefacts can be attributed partly to incoherent (Compton) scattering. Incoherent scattering is uniformly distributed in space and the effects thereof can be removed during reconstruction by subtracting a constant value from the signal measured by each cell in the detector. Said constant value is dependent on the dimensions of the object to be examined. Taking incoherent scatter into account in this manner results in a substantial improvement of the reconstructed image.
However, it has been found that a given degree of unsharpness and artefacts remain in the reconstructed image also after the above correction.